Various thermal transfer recording methods have hitherto been known. Among them, a method has been proposed in which various full-color images are formed using thermal transfer sheets comprising a colorant layer provided on a substrate, the color layer comprising dyes for dye sublimation transfer supported by a suitable binder. Since the colorants used are dyes, images formed using such thermal transfer sheets are very sharp and highly transparent. Thus, the images have excellent halftone reproducibility and gradation equivalent to those of images obtained by conventional offset printing and gravure printing and have a high quality comparable with that of full-color photographic images.
In the formation of images using thermal transfer sheets, a method is generally adopted that comprises providing a printer provided with a linear thermal head comprising heating elements arranged in a row, scanning, in a direction perpendicular to the longitudinal direction of the thermal head, a thermal transfer sheet and an object that have been superimposed on each other so that the surface of a colorant layer in the thermal transfer sheet faces the object, and, in this state, heating the assembly from the substrate surface side to transfer dyes to the object, thereby forming an image.
In thermal transfer sheets, when printing is carried out by bringing the thermal head into direct contact with the substrate, sticking occurs during scanning by a frictional force applied between the substrate and the thermal head, sometimes leading to defective printing. Further, in some cases, the substrate is fused to the thermal head by heat applied in printing, and this fusing hinders the travel of the thermal transfer sheet, disadvantageously leading to sticking and, in a remarkable case, sometimes leading to sheet breaking. In order to prevent these unfavorable phenomena, in thermal transfer sheets, a heat-resistant slipping layer is provided on the substrate in its surface that comes into contact with the thermal head, that is, the surface of the substrate opposite to the colorant layer, from the viewpoints of improving the heat resistance and imparting a slipping property to realize travel stability.
The heat-resistant slipping layer is formed by coating a coating liquid comprising binder resins and slipping agents, such as phosphoric ester-based surfactants, metal soaps, or talc, as a slipping agent dissolved or dispersed in a suitable solvent on a substrate, and drying the coating. For example, Japanese Patent Application Laid-Open No. 61679/2009 (patent document 1) proposes a thermal transfer sheet that has strength and heat resistance of a heat-resistant slipping layer improved through the combined use of a polyvinyl acetal-based resin and a polyisocyanate as binder resins that cause crosslinking between a hydroxyl group in the polyvinyl acetal and an isocyanate group. Further, Japanese Patent Application Laid-Open No. 144852/2005 (patent document 2) proposes a thermal transfer sheet that has strength and heat resistance of a heat-resistant slipping layer improved through the combined use of an acrylic polyol-based resin and a polyisocyanate as binder resins that cause crosslinking between a hydroxyl group in the acrylic polyol resin and an isocyanate group.
The use of the binder resins from the viewpoint of obtaining a highly heat-resistant slipping layer requires heat for the progress of the crosslinking reaction. Accordingly, a step (an aging step) of, after once forming a heat-resistant slipping layer on a substrate, applying heat to the colorant layer (ink ribbon) is necessary. That is, an off-line step should be adopted. Therefore, an in-line process that can simultaneously form the heat-resistant slipping layer and the colorant layer without passage through the aging step cannot be realized, disadvantageously making it impossible to increase the production speed of thermal transfer sheets.